1. Field of the Invention
The present invention relates to a linear transmission device, and more particularly to a linear ball bearing guideway with increased load capacity and stiffness in downward direction.
2. Description of the Prior Art
Linear ball bearing guideway is an important linear transmission device, and has been widely used on different transportation devices because of its advantages of high efficiency and accuracy. Rated static load capacity and stiffness are two important considerations when choosing linear ball bearing guideways, especially, the rated static load capacity and stiffness in the downward direction are very important. The rated static load capacity of a linear ball bearing guideway is usually in direct proportion to the number of rows of balls and the diameter of the balls. Therefore, increasing the ball diameter or the number of row of the balls is a measure usually taken to increase the rated static load capacity. However, the existing linear ball bearing guidways are mostly not designed to strengthen the rated static load capacity and stiffness in the downward direction, therefore, the rated static load capacity and stiffness in the downward direction are generally low.
FIG. 1 shows a linear ball bearing guideway 10 disclosed in U.S. Pat. No. 6,132,093, which is a conventional technology for increasing the rated static load capacity and stiffness in the downward direction. The linear ball bearing guideway 10 generally comprises: a slider 11 slidably mounted on a rail 12, and four rows of balls 13 disposed between the slider 11 and the rail 12. The number of the rows of balls of the linear ball bearing guideway 10 is increased from 2 to 4 as compared with other linear ball bearing guideways, so as to increase the rated static load capacity of the linear ball bearing guideway 10. Two rows of balls 13 are disposed at the top of the rail 12, and another two rows of balls 13 are disposed at two sides of the rail 12. However, by looking at the arrangements of the four rows of balls 13, we can see that the two rows of balls 13 at the top of the rail 12 are only capable of bearing the load in the downward direction, but totally unable to bear any load in the lateral direction, while the two rows of balls 13 at two sides of the rail 12 are able to bear the load only in the lateral direction, but totally unable to bear any of the load in the downward direction. In general, the load capacity cannot be considerably improved in both downward and lateral directions. The load capacity might be improved in the downward direction but not in the lateral direction, as a result, the load capacity in general is still not sufficient to enable the linear ball bearing guideway to be applicable to most machines. Furthermore, due to the fact that both the top surface and lateral surfaces of the rail 12 are provided with rolling grooves for reception of the balls 13, the rail 12 must be subjected to a top grinding process and a lateral grinding process, which substantially increases manufacturing cost and reduces manufacturing efficiency. Besides, the two grinding processes have different grinding datum, which may result in position error of the rolling grooves at the top surface and the lateral surfaces, and reduces the accuracy of the ball bearing guideway.
As shown in FIG. 2, a linear ball bearing guideway 20 which is disclosed in U.S. Pat. No. 8,414,190 also comprises: a slider 21 slidably mounted on a rail 22, and the number of rows of the balls 23 is increased from 2 to 8 as compared with general linear ball bearing guideways. Also due to the fact that both the top surface and lateral surfaces of the rail 22 are provided with four rolling grooves, and the two lateral surfaces are inclined toward the bottom of the rail 22, the rail 22 must be subjected to a top grinding process and a lateral grinding process. Therefore, this linear ball bearing guideway 20 also has the defects of high manufacturing cost, and low manufacturing efficiency and accuracy. Besides, the linear ball bearing guideway 20 is considerably increased in size, because of the arrangement of the 8 rows of balls.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.